One type of rotor aircraft, referred to as a gyroplane, has a fuselage, wings, rotor and a propeller. For a jump takeoff, the rotor is pre-rotated to an overspeed while the aircraft is still on the ground. The propeller also is rotating. The pilot releases the brakes and increases the collective pitch on the propeller and rotor to cause the aircraft to lift from the ground. As the aircraft picks up forward speed, the wings assume more of the load and the rotor is tilted forward, decreasing the rotor speed and decreasing the load assumed by the rotor. At cruise speed, the rotor turns slowly while the wings supply most of the lift.
Changing the responsibilities of the rotor from primary lift source to passive lift source has great application and speed because the wings have much less drag than a fast turning rotor at high speeds. This transfer, however, can be responsible for significant ride vibrations or disturbances felt by the passengers. The oscillations, which occur twice per revolution for a two blade rotor, are due to the fact the rotor produces more lift and drag when the rotor is perpendicular to the airflow than when it is parallel to the airflow. Even when the rotor is substantially unloaded, these oscillations will be observable and made even more noticeable due to the reduced rotor RPM at the higher airspeeds. For large transport aircraft the rotor may be slowed down to 25 RPM during high speed cruise
One way of reducing the disturbances felt by the passengers is by increasing the oscillation frequency by increasing the number of blades, because higher frequencies are not as disruptive to passengers. Typical rotor aircrafts such as helicopters do not slow down the rotor during cruise conditions, rather the blades remain rotating at a high speed. The gyroplane described above, however, is able to obtain high cruising speeds only by slowing down the rotor speed.
Rotor aircraft can increase oscillation frequency and reduce the amplitude by increasing the number of rotor blades. However, in addition to the increased complexity and weight from using more than two blades, there is another significant drawback to using more than two blades on the rotor, which is the inability to have compact storage. When using a three or four bladed rotor, it becomes necessary to fold the blades to store the aircraft compactly. This increases weight and complexity. A two bladed rotor has the ability to store compactly by arranging a rotor blade fore and aft over the aircraft. This eliminates the weight and complexity associated with folding, and allows for a more compact rotor hub housing. This advantage is particularly significant aboard an aircraft carrier where space is limited.